514 PRINCIPLES OF CHEMISTRY 



On heating, the reverse phenomenon is observed. The reaction in the 

 solution depends upon the insolubility of: the lead sulphate, and the 

 decomposition which takes place on heating is due to the volatility 

 of the lead chloride. Silver sulphate, Ag 2 S0 4 , in solution with common 

 salt,, gives silver .chloride, because the latter is insoluble- in water, 

 Ag 2 S0 4 + 2NaCl = Na 2 S0 4 + 2AgCl. Sodium carbonate, mixed in 

 solution with the sulphates of iron, copper, manganese, magnesium, &c., 

 gives in solution sodium sulphate, and in the precipitate a carbonate 

 of the corresponding metal, because these salts of carbonic acid 

 are insoluble in water; for instance, MgS0 4 H- Na^C0 3 == Na 2 SO 4 

 + MgCO 3 . In precisely the same way sodium hydroxide acts on 

 solutions of the majority of the salts of sulphuric acid containing 

 metals, the hydroxides of which are insoluble in water for instance, 

 uS0 4 -f 2NaHO = Cu(HO) 2 4- Na 2 S0 4 . Sulphate of magnesium, 

 MgSO 4 , on being mixed in solution with common sal^ forms, although 

 not completely, chloride of magnesium, and sodium sulphate. On cool- 

 ing the. mixture of such (concentrated) solutions sodium sulphate is 

 deposited, as was shown in Chapter X. This is made use of for prepar- 

 ing it on the large scale in works where sea-water is treated. In this 

 case, on cooling, the reaction 2NaCl + MgS0 4 = MgCl 2 + Na 2 S0 4 

 takes place. 



Thus where sulphates and salts of sodium are in contact, it may 

 \>Q expected that sodium sulphate will be formed and separated if 

 the conditions are favourable; for this reason it is not surprising 

 that .sodium sulphate is often found in the native state. Some of the 

 springs and salt lakes in the steppes_ beyond the Yolga, and in the 

 Caucasus, contain a considerable quantity of sodium sulphate, and yield 

 it by simple evaporation of the solutions. Beds of this salt are also 

 met with; thus at a depth, of only 5" feet, about 38 versts to the 

 east of Tiflis, at the foot of the range of the * Wolfs mane' (Voltchia 

 griva) mountains, a deep stratum of very pure Glauber's salt, 

 Na 2 S0 4) 10H 2 O, has been found. 2 A layer two metres thick of the 

 same salt lies at the bottom of several lakes (an area of about 10 

 square kilometres) in the Kouban district -near Batalpaschinsk, and 

 here its working has been commenced (1 887). In Spain, near Arangoulz 

 and in many parts of the Western States of North America, mineral 

 sodium sulphate has likewise been found, and is already being worked. 



The methods of obtaining salts by means of double decomposition 



3 Anhydrous (ignited) sodium sulphate, Na2S04, is known in trade as ' sulphate ' 

 or salt-cake, in mineralogy thenardite. Crystalline decahydrated salt is termed in 

 mineralogy mirabilite, and in trade Glauber's salt. On fusing it, the monohydrate 

 Na-jSOaHaO is obtained, together with a supersaturated solution. 



